1. Field of the Invention
This invention relates to enclosures for hybrid circuits and methods of manufacture of such enclosures.
2. Brief Description of the Prior Art
Hybrid electrical circuits, especially such circuits of the chip and wire hybrid type, require the protection of a sealed package or enclosure. The function of the hybrid circuit enclosure is to house and protect a ceramic hybrid circuit, either thick-film or thin-film, by providing a hermetically sealed environment for such circuit. The standard enclosures of the above noted type are fabricated from solid metal (i.e., Kovar, metal alloy ASTM F-15 and the like) which is preferred for glass (low expansion) sealing of insulated through-wall electrical connection leads. Several fabrication methods have been used, these including machining of a boat from a solid block of metal, brazing metal walls to a metal base and stamping of the boat shape from the metal being used. Holes are then punched or drilled in the bottom and/or side walls for vertical and/or horizontal pin-outs, as required. Glass is then used in the holes in conjunction with the kovar conductor pins or leads to hermetically seal the leads in the walls of the enclosure after heating to an appropriate temperature.
The prior art of hybrid electric circuit packages and enclosures is set forth in detail in "Power Hybrids Aid HF Supply Design", by Dennis Buchenholz, PCIM, October, 1988, pages 68 to 70, "Hybrid Packages Maximize Circuit Protection" by Howard W. Markstein, Electronic Packaging & Production, January 1989, pages 48 to 51 and "Designing Power Hybrid Packages" by Al Krum, Hybrid Circuit Technology, March, 1989, pages 17 to 24.
The enclosure must also offer efficient heat dissipation, hermeticity minimum size and a reliable means of connection to an external device, such as a printed circuit board.
As is readily apparent from the prior art, when high power devices are mounted inside the hybrid enclosures or packages, either directly on the package material or on a substrate between the device and the enclosure, a high thermal conductivity enclosure material is required to dissipate the heat generated by the electrical circuit. It is also required that this material have a low coefficient of thermal expansion to avoid cracking or other damage to the electrical circuit.
One standard solution to this problem in the past has been to utilize a kovar ring and braze it to a specialty material i.e., molybdenum, copper-tungsten, copper-molybdenum-copper, clad sheet materials, etc.) which is used as the base material onto which the electrical circuit is affixed.
The above described solution has several disadvantages, among them being high cost of the solid base specialty material, the difficulty in brazing of joints of dissimilar metals (i.e., molybdenum to copper) and the requirement that the leads or pins be confined to the kovar sidewalls of the enclosure in order to maintain hermeticity in conjunction with the glass seal.
It is generally known that an effective hybrid enclosure must have a base which displays high thermal conductivity to transfer heat from the electrical hybrid circuit which is secured thereto to the enclosure exterior through the bottom. It is also highly desirable that the enclosure base simultaneously have a controlled, preferably low, coefficient of thermal expansion which matches the coefficient of thermal expansion of the substrate (usually a ceramic material) of the hybrid circuit to be secured within the enclosure. This prevents or minimizes cracking of the substrate of the hybrid circuit due to thermal cycling or thermal shock. It is further desirable that the lid for the enclosure be hermetically weldable to the sidewalls thereof, generally such as by seam welding or other techniques. This is difficult with, for example, copper sidewalls. A still further desirable feature is that there be matched seals as opposed to compression seals to provide the required hermeticity wherein the expansion properties of the glass sealant is matched to the metal forming the enclosure portion where the seal is to be formed. Kovar is the desired material for thermal matching with glass sealant. A yet further desired property is flexural strength of the base of the enclosure to minimize bowing thereof and thereby minimizing the likelihood of the electrical circuit substrate separating from the base or cracking and decreasing thermal conductivity between substrate and base. A still further consideration is economics which is always present. It is therefore readily apparent that the materials of choice, if displaying the required properties, are copper, steel, aluminum and the like which are relatively inexpensive. A yet further consideration is the ability to provide leads or pins through the enclosure both vertically and horizontally. This requires that all surfaces of the enclosure which will retain such leads or pins have the property of sealing hermetically to the bonding material, generally glass. The prior art has long sought but has been unable to provide enclosures for hybrid electrical circuits which meet all of the above criteria.